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#### Quantities and Units of Heat

In thermodynamics, heat is a form of engergy in transfer because of temperature difference. The units to measure amount of heat are equivalent to the units of energy. Among many of units of heat, joule is SI unit of work, energy and amount of heat. However, the most commonly used are probably the calorie and the British thermal unit (BTU).

Since heat is usually studied in heat transfer, the process to transfer from one location to another, the heat related units often cover physical quantities related to the process of heat transfer. For example, heat flux, specific heat, thermal conductivity and the overall heat transfer coefficient.

Heat Flux, or density of heat flow rate, is a quantity to express the amount of heat transferred in a unit time per unit area. The SI unit of heat flux watt per square meter (symbol W/m²).

Specific heat  is the amount of heat per unit mass required to raise the temperature by one degree Celsius. The SI for specific heat is joule per kilogram kelvin (symbol J/(kg·K)).

Thermal conductivity is the quantity of heat transferred per unit time through a surface of unit area due to the exist of certain temperature gradient. The SI unit is watt per meter kelvin (symbol W/(m·K).

The overall heat transfer coefficient describes the rate of heat transfer in the heat exchanger. It expresses the amount of heat which passes through a unit area of a medium or system in a unit time when the temperature difference between the surface and surrounding medium is 1 degree. The SI unit is watt per square meter kelvin (symbol W/(m²·K).